Dinitrogen pentoxide (also known as nitrogen pentoxide or nitric anhydride) is the chemical compound with the formula N2O5. It is one of the binary nitrogen oxides, a family of compounds that contain only nitrogen and oxygen. It exists as colourless crystals that sublime slightly above room temperature, yielding a colorless gas.

Dinitrogen pentoxide has been used for nitrations but has largely been superseded by nitronium tetrafluoroborate (NO2BF4).

Structure

structure of gaseous N2O5

Solid N2O5 is a salt, nitronium nitrate, consisting of linear nitronium cations [NO2]+ and planar trigonal nitrate anions [NO3]−. The N-O distances in the cation are 115 pm and 124 pm for the anion. Gaseous N2O5 is a covalently-bound molecule. It crystallizes in the space group D4 6h (C6/mmc) with Z=2.

In the gas phase or when dissolved in nonpolar solvents such as carbon tetrachloride, the compound exists as covalently-bonded molecules O2N−O−NO2. In the gas phase, theoretical calculations for the minimum-energy configuration indicate that the O−N−O angle in each −NO2 wing is about 134° and the N−O−N angle is about 112°. In that configuration, the two −NO2 groups are rotated about 35° around the bonds to the central oxygen, away from the N−O−N plane. The molecule thus has a propeller shape, with one axis of 180° rotational symmetry (C2)

Physical properties

The vapor pressure P (in atm) as a function of temperature T (in kelvin), in the range 211 to 305K (−62 to 32°C), is well approximated by the formula

ln ⁡ P = 23.2348 − 7098.2 T {\displaystyle \ln P=23.2348-{\frac {7098.2}{T}}}

being about 48 torr at 0°C, 424 torr at 25°C, and 760 torr at 32°C (9°C below the melting point).

When gaseous N2O5 is cooled rapidly ("quenched"), one can obtain the metastable molecular form, which exothermically converts to the ionic form above −70°C.

Gaseous N2O5 absorbs ultraviolet light with dissociation into the free radicals nitrogen dioxide NO2• and nitrogen trioxide NO3• (uncharged nitrate). The absorption spectrum has a broad band with maximum at wavelength 160nm.

Preparation

N2O5 was first reported by the French chemist Henri Deville in 1840, who prepared it by treating silver nitrate (AgNO3) with chlorine.

A recommended laboratory synthesis entails dehydrating nitric acid (HNO3) with phosphorus(V) oxide:

P4O10 + 12 HNO3 → 4 H3PO4 + 6 N2O5

Another laboratory process is the reaction of lithium nitrate LiNO3 and bromine pentafluoride BrF5, in the ratio exceeding 3:1. The reaction first forms nitryl fluoride FNO2 that reacts further with the lithium nitrate:

BrF5 + 3 LiNO3 → 3 LiF + BrONO2 + O2 + 2 FNO2

FNO2 + LiNO3 → LiF + N2O5

The compound can also be created in the gas phase by reacting nitrogen dioxide NO2 or N2O4 with ozone:

2 NO2 + O3 → N2O5 + O2

However, the product catalyzes the rapid decomposition of ozone:

2 O3 + N2O5 → 3 O2 + N2O5

Dinitrogen pentoxide is also formed when a mixture of oxygen and nitrogen is passed through an electric discharge. Another route is the reactions of phosphoryl chloride POCl3 or nitryl chloride NO2Cl with silver nitrate AgNO3

Reactions

Sublimed dinitrogen pentoxide crystals

Dinitrogen pentoxide reacts with water (hydrolyses) to produce nitric acid HNO3. Thus, dinitrogen pentoxide is an acidic oxide, the anhydride of nitric acid:

N2O5 + H2O → 2 HNO3

Solutions of dinitrogen pentoxide in nitric acid can be seen as nitric acid with more than 100% concentration. The phase diagram of the system H2O−N2O5 shows the well-known negative azeotrope at 60% N2O5 (that is, 70% HNO3), a positive azeotrope at 85.7% N2O5 (100% HNO3), and another negative one at 87.5% N2O5 ("102% HNO3").

The reaction with hydrogen chloride HCl also gives nitric acid and nitryl chloride NO2Cl:

N2O5 + HCl → HNO3 + NO2Cl

Dinitrogen pentoxide reacts with ammonia NH3 to give several products, including nitrous oxide N2O, ammonium nitrate NH4NO3, nitramide NH2NO2, and ammonium dinitramide NH4N(NO2)2, depending on reaction conditions.

Thermal decomposition

Decomposition is negligible if the solid is kept at 0°C. It decomposes at room temperature into NO2 and O2:

N2O5 → 2 NO2 + 0.5 O2

Its solutions in carbon tetrachloride decompose at 30°C (303K). Both N2O5 and NO2 are soluble in CCl4 and remain in solution while oxygen is insoluble and escapes. The volume of the oxygen allows determination of the reaction kinetics:

2 N2O5 → 4 NO2 + O2(g)

In the presence of nitric oxide, the decomposition proceeds as follows:

N2O5 + NO → 3 NO2

Applications

Nitration of organic compounds

Dinitrogen pentoxide, for example as a solution in chloroform, has been used as a reagent to introduce the −NO2 functionality in organic compounds. This nitration reaction is represented as follows:

N2O5 + Ar−H → HNO3 + Ar−NO2

where Ar represents an arene moiety. The reactivity of the NO+2 can be further enhanced with strong acids that generate the superelectrophile HNO2+2.

In this use, N2O5 has been largely replaced by nitronium tetrafluoroborate [NO2]+[BF4]−. This salt retains the high reactivity of NO+2, but it is thermally stable, decomposing at about 180°C (into NO2F and BF3).

Dinitrogen pentoxide is relevant to the preparation of explosives.

Atmospheric occurrence

In Earth's atmosphere, dinitrogen pentoxide is an important reservoir of the NOx species that are responsible for ozone depletion: its formation provides a null cycle with which NO and NO2 are temporarily held in an unreactive state. Mixing ratios of several parts per billion by volume have been observed in polluted regions of the nighttime troposphere. Dinitrogen pentoxide has also been observed in the stratosphere at similar levels, the reservoir formation having been postulated in considering the puzzling observations of a sudden drop in stratospheric NO2 levels above 50°N, the so-called 'Noxon cliff'.

Variations in N2O5 reactivity in aerosols can result in significant losses in tropospheric ozone, hydroxyl radicals, and NOx concentrations. Two important reactions of N2O5 in atmospheric aerosols are hydrolysis to form nitric acid and reaction with halide ions, particularly Cl−, to form ClNO2 molecules which may serve as precursors to reactive chlorine atoms in the atmosphere.

Hazards

N2O5 is a strong oxidizer that forms explosive mixtures with organic compounds and ammonium salts. Dinitrogen pentoxide can degrade to the highly toxic nitrogen dioxide gas.

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